128 A HUMAN EMBRYO OF TWENTY-FOUR PAIRS OF SOMITES. 



through the graded alcohols, cleared in chloroform, and embedded in paraffin. It 

 was cut in sections 8 microns thick and stained with alum hemotoxylin and eosin. 

 An idea of the plane of sectioning can be obtained from text-figure 1. 



The age of the specimen is uncertain from the data obtained. As determined 

 from a model of the embryo, its greatest length is about 2.4 mm. This measure- 

 ment and those of the chorionic vesicle make it correspond very closely to an 

 embryo of 28 days, as estimated by Mall 32 , p. 199). If such is its age the specimen 

 falls into that group of embryos which continue to develop in the uterus during a 

 menstrual flow. 



Comparison with descriptions of other embryos shows that my specimen is 

 near the age of Robert Meyer's embryo No. 300, described by Thompson 45 ; it 

 resembles also Janosik's specimen 20 and His's embryo "Lg." 18 . In most respects, 

 however, it is younger than the above-named specimens and also younger than the 

 embryos described by Mall 29 , Gage 12 , Bremer 2 , and Tandler 44 . It is apparently 

 older than the twin specimens of 17-19 segments described by Watt 48 . 



A glance at the figure (plate 1, fig. 1) of the embryo is sufficient to show that 

 there is a marked ventral bend in its back, such as has been found in many speci- 

 mens of comparable age. Since this ventral flexure is not invariably present, and 

 varies in degree, it is regarded by Keibel 22 as essentially abnormal. In my embryo 

 it occurs without rupturing the underlying structures; if an abnormality, it is the 

 only one which seems to be present. 



I am indebted to my former students, Messrs. W. L. Brosius, L. B. Hohman, 

 H. L. Houchins, L. H. Rutledge, Florien Vaughn, and T. F. Wheeldon, for certain 

 reconstructions which have aided me greatly in my study of the embryo. To 

 these men and also to Mr. G. T. Kline, who has made many of the illustrations, 

 I wish to express my sincere thanks. I wish also to express my gratitude to Pro- 

 fessor Frederic T. Lewis, who has read my manuscript and offered many valuable 

 criticisms. 



EXTERNAL FORM. 



The external form of the embryo has been studied from a wax reconstruction 

 which was made at a magnification of 120 diameters. As already stated, the em- 

 bryo was cut up without being previously drawn. It was necessary, therefore, to 

 use the walls of the chorionic sac as guide-lines in making the model. 



As seen from the left side (plate 1, fig. 1), the embryo is roughly the shape of a 

 reversed S. Its back presents two well-developed curves. The upper of these is con- 

 vex dorsally; it is large and rounded. The lower curve is convex ventrally, since 

 the caudal portion of the embryo is bent sharply backward at about right angles to 

 its longitudinal axis. Not only is the caudal end of the embryo bent backward 

 upon itself, but at the point of bending it is twisted through an angle of 90 to the 

 right. Thus the dorsal wall of this portion of the embryo is turned to the right 

 and the ventral wall to the left; also, the caudal end of the embryo is directed to 

 the left, so that its tip lies to the left of the axis of the body of the embryo. 



